AspNet Core External Authentication with Both Google and Facebook - asp.net-core

Question

I am trying to implement the Form-Authentication in ASP.Net Core with Both Google and Facebook Authentications. I followed some tutorials and after some struggles, I managed to make it work both.
However, the problem is that I cannot use both authentications for the same email.
For example, my email is 'ttcg#gmail.com'.
I used Facebook authentication to log in first... Registered my email and it worked successfully and put my record into 'dbo.ASPNetUsers' table.
Then I logged out, clicked on Google Authentication to log in. It authenticated successfully, but when I tried to register it keeps saying that my email is already taken.
I tried to do the same thing for other online websites (Eg, Stackoverflow). I used the same email for both Google and Facebook and the website knows, I am the same person and both my login / claims are linked even though they come from different places (Google & Facebook).
I would like to have that feature in my website and could you please let me know how could I achieve that.
In theory, it should put another line in 'dbo.AspNetUserLogins' and should link the same UserId with multiple logins.
Do I need to implement my own SignInManager.SignInAsync method to achieve that feature? Or am I missing any configuration?

Answer 1

You need to link your Facebook external login to your Google external login with your email by using UserManager.AddLoginAsync, you cannot register twice using the same adresse if you use the adresse as login.
Check out the Identity sample on Identity github repo.
https://github.com/aspnet/Identity/blob/dev/samples/IdentitySample.Mvc/Controllers/ManageController.cs
To link external login to a user, the Manae controller expose methods LinkLogin and LinkLoginCallback
LinkLogin requests a redirect to the external login provider to link a login for the current user
LinkLoginCallback processes the provider response
//
// POST: /Manage/LinkLogin
[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult LinkLogin(string provider)
{
// Request a redirect to the external login provider to link a login for the current user
var redirectUrl = Url.Action("LinkLoginCallback", "Manage");
var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirectUrl, _userManager.GetUserId(User));
return Challenge(properties, provider);
}
//
// GET: /Manage/LinkLoginCallback
[HttpGet]
public async Task<ActionResult> LinkLoginCallback()
{
var user = await GetCurrentUserAsync();
if (user == null)
{
return View("Error");
}
var info = await _signInManager.GetExternalLoginInfoAsync(await _userManager.GetUserIdAsync(user));
if (info == null)
{
return RedirectToAction(nameof(ManageLogins), new { Message = ManageMessageId.Error });
}
var result = await _userManager.AddLoginAsync(user, info);
var message = result.Succeeded ? ManageMessageId.AddLoginSuccess : ManageMessageId.Error;
return RedirectToAction(nameof(ManageLogins), new { Message = message });
}